Investigation On Numerical Simulation And Optical Diagnosing Of Helicon Discharge Plasmas In Electric Propulsion

With the advantages of high ionization efficiency,wide propellant choice,high specific impulseandno erosion byelectrodeless dicharge,the heliconplasma thruster has attracted much attention in the field of electric propulsion.This thesis employs both the method of numerical simulation and optical diagnosing to investigate the helicon discharge plasma:a direct numerical simulation model of helicon plasma discharge with three dimensional fluid-dynamics equations is proposed,and then plasma parameters are measured by optical emission spectrum?OES?and laser induced fluorescence?LIF?.The present numerical model takes detailed consideration of electrochemical reactions and collision relations.Itdiscards the small perturbation theory in previous helicon models which are partially analytical in essence.In the assumption of weak ionization,the Maxwell equations coupled with the plasma parameters are directly solved.Thus the energy deposited from electromagnetic wave to plasma can be then easily calculated.The values of plasma parameters which include electron density,mean electron energy and heavy species density were obtained by solving a set of drift-diffusion equations.All of the partial differential equations are solved by the finite element solver of COMSOL Multiphysics^TM with the full coupled method.The numerical model can led to an identical dispersion relationof helicon wave and T-G wave as the classical wave theory by analysis.Besides,the numerical results match well with the typical experimental values and static sheath theory.Further more,several concerned aspects in the helicon discharge plasma are investigated by this model and some significative conclusions are arrived:rational explanations about the spacialdistribution of partical densities are provided by the govern equations and boundary condtions in this work;the mechanism of the low-field density peak in helicon discharge is validated as the wave interference with thesubstanceof standing wave generated by the helicon mode and its reflected wave from endplates;moreover,numerical results show different characteristics on temporal evolution of particle density and electron temperature in helicon discharge,whichgives more emphasis on the comprehension of the ion pumping effect and electron heating mechanism.A modified and simplified collisional-radiative model?CRM?based on the detailed term accounting approximation is proposed for the argon plasma at low electron temperature.The terms up to the main quantum number N=10 aretaken into accountand divided into 47 effective levels to ensure both of the precision and efficiency in calculation.Unlike the high temperature plasma at hundreds of electron volts,the ratios of calculated line intensities depend both on the electron temperature and density for low temperature plasma,which are especially sensitive below 6eV.Thus the traditional diagnosing method of line ratio comparing is not adaptable for plasma at low temperature.For a more precise measurement,the method of spectrum matching was employed to plasma diagnosing.The 15 lines which are cognizable within the wavelength from 680nm to 860nm are utilized to match with the results calculated by the collisional-radiative model.Intensities calculated by this model have an average error of 13.7%.A twice matching process based on the screening of a priori knowledge is put forward to diagnose the plasma,in which the additional matching process makes use of the absolute values of the measured line intensities.Results indicate that the precision of the electron temperature and density has been improved a lot and the relative errors are 25%and 40%,respectively.An LIF experiment system for measuring the ion velocity precisely is established in this work,which utilizes the phase-sensitive detectorto isolate weak fluorescence signal.A Gaussian inverse filter is employed to obtainthe pure Doppler broadening and shift which are thenthransformed to theion velocity distribution function?IVDF?.A bidirectional polarization scheme is proposed to measure the argon IVDF near the exit of plasma source.Results show the ion does not reach a velocity caused by electric double layer?EDL?.Finally,the LIF system is employed to measure the xeon ion velocity,temperature,plume divergence angle,energy efficiency and the orther comprehensive parameters in a Hall effect thruster for further verifying the performance and extending the applicability.By these investigations,the gas discharge dynamic process and the effect of discharge parameters on the plasma generation efficiency are comprehended more profoundly in helicon discharge plasma.Thus they can support the designing and establishing for helicon source.Moreover,the controversial compact helicon plasma thruster by EDL is experimently proved negative.